Learning Objectives
- Understand the cell theory and its modern interpretation
- Differentiate between prokaryotic and eukaryotic cells
- Learn the structure and functions of cell organelles
- Understand the structure of the plasma membrane, nucleus, and endomembrane system
Key Concepts
Cell Theory
Proposed by Schleiden and Schwann (1838-39): All living organisms are composed of cells and products of cells. Rudolf Virchow added: All cells arise from pre-existing cells (Omnis cellula-e cellula). Robert Hooke (1665) discovered cells in cork. Leeuwenhoek first observed live cells (bacteria, RBCs).
Prokaryotic vs Eukaryotic Cells
Prokaryotic: No true nucleus (nucleoid region), no membrane-bound organelles, 70S ribosomes, single circular DNA (naked), cell division by fission. Examples: bacteria, cyanobacteria, mycoplasma. Eukaryotic: True nucleus with nuclear membrane, membrane-bound organelles, 80S ribosomes (70S in mitochondria and chloroplasts), linear DNA with histones, cell division by mitosis/meiosis.
Plasma Membrane
Fluid Mosaic Model by Singer and Nicolson (1972): Membrane is a phospholipid bilayer with proteins (integral/peripheral) and cholesterol. Lipids provide fluidity; proteins perform transport, enzymatic activity, and signal transduction. Selectively permeable. Transport: passive (diffusion, osmosis, facilitated diffusion â no ATP) and active (against concentration gradient â requires ATP, e.g., Na-K pump).
Cell Wall
Present in plant cells, fungi, and bacteria. In plants: made of cellulose (microfibrils). Middle lamella (calcium pectate) holds adjacent cells together. Plasmodesmata are cytoplasmic connections between adjacent plant cells through cell wall.
Endomembrane System
Includes ER, Golgi, lysosomes, vacuoles. Endoplasmic Reticulum: Rough ER (with ribosomes â protein synthesis) and Smooth ER (lipid synthesis, detoxification). Golgi Apparatus (Dictyosome): Cis face (forming/receiving face) and Trans face (maturing/secreting face). Packaging, modification, transport of proteins. Forms lysosomes. Lysosomes: Contain hydrolytic enzymes; called "suicidal bags" (can digest own cell). Vacuoles: Large central vacuole in plant cells (tonoplast membrane) maintains turgor pressure.
Other Organelles
Mitochondria: Double membrane, inner membrane folded into cristae, matrix contains 70S ribosomes, circular DNA, enzymes for Krebs cycle. Called "powerhouse of the cell" (ATP production). Semi-autonomous organelle.
Plastids: Only in plant cells. Chloroplasts (green â photosynthesis; double membrane, thylakoids stacked into grana, stroma with 70S ribosomes and circular DNA), Chromoplasts (coloured â carotenoids), Leucoplasts (colourless â storage: amyloplasts for starch, elaioplasts for oil, aleuroplasts for protein).
Ribosomes: Non-membrane bound. 70S in prokaryotes (50S + 30S); 80S in eukaryotes (60S + 40S). Site of protein synthesis.
Centrosome: Contains two centrioles (9+0 arrangement of microtubule triplets). Present in animal cells. Forms spindle fibres during cell division.
Nucleus: Double membrane with nuclear pores. Contains chromatin (DNA + histones). Nucleolus â site of rRNA synthesis. Nuclear pores allow transport between nucleus and cytoplasm.
Cilia and Flagella: 9+2 arrangement of microtubules (axoneme). Cilia are short and numerous; flagella are long and few. Basal body has 9+0 arrangement (like centriole).
Summary
Cells are the fundamental units of life. Prokaryotic cells lack a true nucleus and membrane-bound organelles. Eukaryotic cells have well-organized organelles. The plasma membrane follows the Fluid Mosaic Model. The endomembrane system coordinates intracellular transport. Mitochondria and chloroplasts are semi-autonomous organelles with their own DNA.
Important Terms
- Fluid Mosaic Model: Model of membrane structure with lipid bilayer and floating proteins
- Cristae: Inner membrane folds of mitochondria increasing surface area
- Grana: Stacks of thylakoids in chloroplasts where light reactions occur
- Nucleoid: Region of prokaryotic cell containing naked DNA
- Plasmodesmata: Cytoplasmic connections through plant cell walls
- Axoneme: Core of cilia/flagella with 9+2 microtubule arrangement
- Semi-autonomous: Organelles with own DNA and ribosomes (mitochondria, chloroplasts)
- Tonoplast: Membrane surrounding plant cell vacuole
Quick Revision
- Cell theory: Schleiden & Schwann; modified by Virchow
- Prokaryotic = 70S ribosomes, no nuclear membrane; Eukaryotic = 80S, nuclear membrane
- Fluid Mosaic Model by Singer & Nicolson (1972)
- Mitochondria: powerhouse, double membrane, cristae, own DNA (70S ribosomes)
- Chloroplast: thylakoids â grana; stroma; own DNA
- Lysosomes = suicidal bags; Golgi = packaging & secretion
- Cilia/Flagella: 9+2 arrangement; Centriole/Basal body: 9+0
- RER = protein synthesis; SER = lipid synthesis and detoxification